**1. Introduction**

Nowadays, the prevalence of the circular economy spirit requires the replacement of conventional packaging plastics, which are derived from fossil resources, with biopolymers which are produced via the valorization of food and/or agricultural by-products [1,2]. Moreover, the greenhouse effect imposes the turn to more environmentally friendly activities in all life sectors. Finally, the food shortage and rising prices could be handled via the extension of food shelf-life. Under this spirit, this study aimed at the valorization of some food byproducts and the use of natural biodegradable raw materials to improve the preservation of foods. Some of the most frequently used and promising biopolymers for packaging applications are cellulose, starch, gelatin, and chitosan [3–7]. Chitosan is produced by the deacetylation reaction of chitin. Chitin can be extracted via chemical or biotechnological processes from seafood waste such as shrimp, lobster, and crayfish shells [7–10]. Chitosan has been extensively studied as a promising biopolymer to be used in active packaging films, coatings, and other industrial applications due to its antioxidant and antimicrobial properties [11–16]. Its poor mechanical properties can be

**Citation:** Salmas, C.E.; Giannakas, A.E.; Moschovas, D.; Kollia, E.; Georgopoulos, S.; Gioti, C.; Leontiou, A.; Avgeropoulos, A.; Kopsacheili, A.; Avdylaj, L.; et al. Kiwi Fruits Preservation Using Novel Edible Active Coatings Based on Rich Thymol Halloysite Nanostructures and Chitosan/Polyvinyl Alcohol Gels. *Gels* **2022**, *8*, 823. https:// doi.org/10.3390/gels8120823

Academic Editor: Annarosa Gugliuzza

Received: 25 October 2022 Accepted: 9 December 2022 Published: 13 December 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

enhanced by blending with other polymers [17,18] biopolymers [19–21] and/or incorporating nano-reinforcements such as nanoclays to give promising chitosan-based biopolymeric nanocomposite gels ready for film preparation [22]. Due to CS's water swelling, it has lower water barrier properties than other packaging materials but is well known for its good gas barrier properties [23].

Moreover, modern food packaging technologies follow the incorporation of natural preservatives, antioxidants, and antimicrobials such as essential oils (EOs) into the polymer matrix of the biopolymer, targeting the development of gels exhibiting controlled release properties of EOs into the food, and sequentially to the increase of food self-life and food safety [24–29]. Various procedures have been developed to protect the antioxidant and antimicrobial activity of EOs. One of them suggests the encapsulation of EOs into microemulsion or nanoemulsion nanostructures [30–32]. Another one suggests the adsorption of such EOs into cheap and naturally abundant adsorbents such as nanoclays and zeolites [33–36]. The excellent gas barrier properties combined with the intermediate moisture barrier can reduce fruits respiration rate without interrupting the supply of moisture on them. This makes CS a promising coating material to extend the shelf-life of fruits [37].

The kiwifruit is cultivated in many places in Greece. However, Pieria remains the main area of production [38]. Kiwifruit is unique because of its high nutritional content [39]. In our days there is a major effort from researchers and farmers to find low cost ways to produce, keep in storage, and deliver onto the market, fruits of high quality [38].

During our previous work, we developed a chitosan/polyvinyl alcohol (CS/PVOH) gel which led to composite films and exhibited improved mechanical, gas barrier, and antimicrobial properties compared to the relevant properties of pure CS film [18,40]. Furthermore, we developed a procedure for the adsorption of EOs such as thyme, oregano, and basil oil in montmorillonite and organophilic montmorillonite. The incorporation of these nanohybrids in Low-Density Poly-Ethylene (LDPE), Polystyrene (PS), and Poly-Lactide-Acid (PLA) active packaging films was also studied previously [27,36,41–43]. A modified method for the adsorption of a fraction rich in thymol from thyme oil (TO) in halloysite nanotube nanoclay (HNT) was applied in this work. The obtained TO@HNT nanohybrids dispersed at 5, 10, and 15 wt.%. nominal content into CS/PVOH matrix via a solution blending method. Pure HNT was also dispersed into CS/PVOH matrix at the same nominal contents and used as reference material. The obtained CS/PVOH/HNT and CS/PVOH/TO@HNT films were characterized via XRD analysis, FTIR spectroscopy, and SEM microscopy. They were also tested for their tensile properties, water/oxygen barrier properties, antioxidant activity, and antimicrobial capacity against food pathogens. Finally, the most active films are applied as a coating to enhance the preservation of kiwi fruits.
